An UPS can continuously provide power supply to electrical equipments, and has been widely researched and utilized. The UPS can be classified into three types according to its operating mode: back-up type, line type and line interactive type.
The line interactive UPS directly provides mains supply to load when the mains supply is normal, and provides power to the load by a battery during the failure of the mains supply. If the voltage of the mains supply is relatively high or low, after being converted by an inverter, the output voltage of the UPS will be higher or lower than a the required voltage for the load. In order to provide a stable output voltage of the line interactive UPS, an automatic voltage regulator (AVR) is usually connected to the input for the mains supply, in order to regulate the output voltage of the UPS. The automatic voltage regulator has high reliability and wide adjustment range for the input voltage.
However, the automatic voltage regulator has a large size, a large weight, high cost and energy consumption, which directly lead to reduction of electrical energy utilization efficiency and increased cost. Therefore, it is desirable to provide an UPS which can provide stable AC voltage without the need of an AVR.